BuddyPress.org

I used it on WordPress, not WordPress MU, however I had tried for short time just to see if it registers users on WordPress MU, and It did, but further than that I have no knowledge of it.

On WordPress, it works fine, just for regular plugin search install gigya and good to go, It will register users fine and allow them to create profiles , wire, messages all stuff. Just this avatar like facebook connect, gravatar brings in wordpress, Gigya is not able to do default way, however in gigya login widget it does show it. Just minor tweaks guess it will require and good to go.